U.S. patent number 9,525,199 [Application Number 14/623,015] was granted by the patent office on 2016-12-20 for millimeter waveband filter.
This patent grant is currently assigned to ANRITUS CORPORATION. The grantee listed for this patent is ANRITSU CORPORATION. Invention is credited to Takashi Kawamura, Hiroshi Shimotahira.
United States Patent |
9,525,199 |
Kawamura , et al. |
December 20, 2016 |
Millimeter waveband filter
Abstract
In a millimeter waveband filter, electric wave half mirrors are
provided in transmission lines of a first waveguide configured to
allow electromagnetic waves in a predetermined frequency range of a
millimeter waveband to propagate in a TE10 mode and a second
waveguide connected to the first waveguide in a state where one end
of the second waveguide is inserted into the first waveguide, and
the waveguides are relatively moved to vary the interval between
the electric wave half mirrors, thereby changing a resonance
frequency. The first waveguide is a square waveguide, and the
second waveguide is a ridge waveguide in which the outside thereof
is a rectangular shape at a predetermined interval with respect to
the inside of the first waveguide and a sectional shape of a
transmission line has a central portion having a height smaller
than both side portions.
Inventors: |
Kawamura; Takashi (Kanagawa,
JP), Shimotahira; Hiroshi (Kanagawa, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
ANRITSU CORPORATION |
Atsugi-shi, Kanagawa |
N/A |
JP |
|
|
Assignee: |
ANRITUS CORPORATION
(Atsugi-Shi, JP)
|
Family
ID: |
54010273 |
Appl.
No.: |
14/623,015 |
Filed: |
February 16, 2015 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20150263399 A1 |
Sep 17, 2015 |
|
Foreign Application Priority Data
|
|
|
|
|
Mar 14, 2014 [JP] |
|
|
2014-051102 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01P
1/208 (20130101); H01P 5/024 (20130101) |
Current International
Class: |
H01P
1/207 (20060101); H01P 1/208 (20060101); H01P
5/02 (20060101) |
Field of
Search: |
;333/209 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Jones; Stephen E
Assistant Examiner: Outten; Scott S
Attorney, Agent or Firm: Pearne & Gordon LLP
Claims
What is claimed is:
1. A millimeter waveband filter comprising: a first waveguide; a
second waveguide, the second waveguide being connected to the first
waveguide in a state where one end of the second waveguide is
inserted into the first waveguide to constitute a transmission line
allowing electromagnetic waves in a predetermined frequency range
of a millimeter waveband to propagate in a TE10 mode; a pair of
flat electric wave half mirrors which have characteristics to
transmit a part of the electromagnetic waves in the predetermined
frequency range and to reflect a part of the electromagnetic waves
and are arranged to face each other at an interval in a state where
the first waveguide and the second waveguide are blocked; and
variable interval means for relatively moving the first waveguide
and the second waveguide in a longitudinal direction of the
transmission line in a state connected together to change the
interval between the pair of electric wave half mirrors, wherein
frequency components centering on a resonance frequency of a
resonator formed between the pair of electric wave half mirrors are
selectively transmitted, the first waveguide is a rectangular
waveguide in which the sectional shape of the inside of the first
waveguide is a rectangular shape such that a lower limit frequency
of a propagatable frequency band of the first waveguide in the TE10
mode is equal to or less than a lower limit of the predetermined
frequency range, the second waveguide is a ridge waveguide in which
the outside thereof is a rectangular shape at a predetermined
interval with respect to the inside of the first waveguide and the
sectional shape of the inside thereof has a central portion having
a height smaller than the height of both side portions, and the
height and width of each of both side portions and the central
portion are set such that a lower limit frequency of a propagatable
frequency band of the second waveguide in the TE10 mode is equal to
or less than the lower limit frequency of the propagatable
frequency band of the first waveguide.
2. The millimeter waveband filter according to claim 1, wherein a
groove which has a length along the longitudinal direction of the
transmission line corresponding to a 1/4 wavelength of
electromagnetic waves to be a leakage prevention target is formed
on the outside of the second waveguide facing the inside of the
first waveguide, and leakage of the electromagnetic waves from the
interval between the outside of the second waveguide and the inside
of the first waveguide is prevented by the groove.
3. The millimeter waveband filter according to claim 2, wherein the
pair of electric wave half mirrors respectively have rectangular
plates which have a predetermined thickness and reflect
electromagnetic waves propagating through the transmission line,
and have slits which are formed in central portions of the plates
along a long side direction of the plates and transmit a part of
the electromagnetic waves propagating through the transmission
line, the slits are of a ridge type in which a central portion has
a height smaller than both side portions, and the thickness of the
plates and the height and width of each of both side portions and
the central portion of the slits are set such that transmittance to
the electromagnetic waves propagating through the transmission line
becomes flat in the predetermined frequency range, and the
predetermined frequency range is 75 to 110 GHz.
4. The millimeter waveband filter according to claim 3, wherein the
sectional shape of the inside of the second waveguide is linearly
symmetrical with a center line in a height direction and a center
line in a width direction of the rectangular shape of the outside
of the second waveguide.
5. The millimeter waveband filter according to claim 4, wherein the
groove is provided in a plurality of stages with respect to a
length direction of the transmission line.
6. The millimeter waveband filter according to claim 5, wherein the
groove has a width of about 1 mm and a depth of about 0.2 mm.
7. The millimeter waveband filter according to claim 4, wherein the
groove has a width of about 1 mm and a depth of about 0.2 mm.
8. The millimeter waveband filter according to claim 3, wherein the
groove is provided in a plurality of stages with respect to a
length direction of the transmission line.
9. The millimeter waveband filter according to claim 8, wherein the
groove has a width of about 1 mm and a depth of about 0.2 mm.
10. The millimeter waveband filter according to claim 3, wherein
the groove has a width of about 1 mm and a depth of about 0.2
mm.
11. The millimeter waveband filter according to claim 2, wherein
the sectional shape of the inside of the second waveguide is
linearly symmetrical with a center line in a height direction and a
center line in a width direction of the rectangular shape of the
outside of the second waveguide.
12. The millimeter waveband filter according to claim 11, wherein
the groove is provided in a plurality of stages with respect to a
length direction of the transmission line.
13. The millimeter waveband filter according to claim 12, wherein
the groove has a width of about 1 mm and a depth of about 0.2 mm,
and the predetermined frequency range is 75 to 110 GHz.
14. The millimeter waveband filter according to claim 11, wherein
the groove has a width of about 1 mm and a depth of about 0.2 mm,
and the predetermined frequency range is 75 to 110 GHz.
15. The millimeter waveband filter according to claim 2, wherein
the groove is provided in a plurality of stages with respect to a
length direction of the transmission line.
16. The millimeter waveband filter according to claim 15, wherein
the groove has a width of about 1 mm and a depth of about 0.2 mm,
and the predetermined frequency range is 75 to 110 GHz.
17. The millimeter waveband filter according to claim 2, wherein
the groove has a width of about 1 mm and a depth of about 0.2 mm,
and the predetermined frequency range is 75 to 110 GHz.
18. The millimeter waveband filter according to claim 1, wherein
the pair of electric wave half mirrors respectively have
rectangular plates which have a predetermined thickness and reflect
electromagnetic waves propagating through the transmission line,
and have slits which are formed in central portions of the plates
along a long side direction of the plates and transmit a part of
the electromagnetic waves propagating through the transmission
line, the slits are of a ridge type in which a central portion has
a height smaller than both side portions, and the thickness of the
plates and the height and width of each of both side portions and
the central portion of the slits are set such that transmittance to
the electromagnetic waves propagating through the transmission line
becomes flat in the predetermined frequency range, and the
predetermined frequency range is 75 to 110 GHz.
19. The millimeter waveband filter according to claim 18, wherein
the sectional shape of the inside of the second waveguide is
linearly symmetrical with a center line in a height direction and a
center line in a width direction of the rectangular shape of the
outside of the second waveguide.
20. The millimeter waveband filter according to claim 1, wherein
the sectional shape of the inside of the second waveguide is
linearly symmetrical with a center line in a height direction and a
center line in a width direction of the rectangular shape of the
outside of the second waveguide.
Description
TECHNICAL FIELD
The present invention relates to a filter which is used in a
millimeter waveband.
BACKGROUND ART
In recent years, there is an increasing need for the use of
electric waves in response to a ubiquitous network society, and a
wireless personal area network (WPAN) which realizes wireless
broadband at home or a millimeter waveband wireless system, such as
a millimeter-wave radar, which supports safe and secure driving
starts to be used. An effort to realize a wireless system at a
frequency equal to or greater than 100 GHz is actively made.
In regard to second harmonic evaluation of a wireless system in a
60 to 70 GHz band or evaluation of a radio signal in a frequency
band equal to or greater than 100 GHz, as the frequency becomes
high, the noise level of a measurement device and conversion loss
of a mixer increase and frequency precision is lowered. For this
reason, a high-sensitivity and high-precision measurement
technology of a radio signal over 100 GHz has not been established.
In the conventional measurement technology, it is not possible to
separate harmonics of local oscillation from the measurement
result, and there is difficulty in strict measurement of
unnecessary emission or the like.
In order to overcome the problems in the related art and to realize
high-sensitivity and high-precision measurement of a radio signal
in a frequency band equal to or greater than 100 GHz, it is
necessary to develop a narrowband filter technology of a millimeter
waveband for the purpose of suppressing an image response and a
high-order harmonic response, and in particular, there is a demand
for a technology which is adaptable to a variable frequency type
(tunable).
In order to realize this, the applicant has suggested a millimeter
waveband filter in which a Fabry-Perot resonator used in an optical
field is applied to millimeter waves and desired frequency
components of the millimeter waves are selectively transmitted by a
resonance action between a pair of electric wave half mirrors
arranged to face each other inside a transmission line allowing
propagation in a TE10 mode (single mode) (Patent Document 1).
RELATED ART DOCUMENT
Patent Document
[Patent Document 1] JP-A-2013-138401
SUMMARY OF THE INVENTION
Problem that the Invention is to Solve
Patent Document 1 described above discloses a structure in which a
transmission line allowing electromagnetic waves in a desired
frequency band to propagate in a TE10 mode is constituted by a
first waveguide having a rectangular sectional shape and a second
waveguide having a rectangular sectional shape with one end thereof
inserted into the first waveguide at a slight interval, electric
wave half mirrors are provided inside the first waveguide and at
the leading end of the second waveguide, and the other waveguide is
relatively moved in a longitudinal direction with respect to one
waveguide to change the gap.
In this structure, the size of the second waveguide inserted into
the first waveguide is inevitably smaller than the size of the
first waveguide by the thickness of the second waveguide and the
interval between the waveguides necessary for movement and a
propagatable frequency range in the TE10 mode is different
according to the size difference. Accordingly, when the waveguides
having a rectangular sectional shape described above are used, it
is a prerequisite that the waveguides are used in a region where
the propagatable frequency ranges in the TE10 mode determined by
the sizes of both waveguides overlap each other.
For example, even when a generally known WR-10 waveguide having a
size of 2.54.times.1.27 mm is used as the outer first waveguide,
the required minimum thickness of the second waveguide is about 0.1
mm, and the interval between both waveguides is 30 .mu.m, the size
(the sectional shape of the inside) of the second waveguide becomes
2.28.times.1.01 mm, a lower limit frequency of a propagatable
frequency domain in the TE10 mode increases by a decrease in size,
and a low frequency band is narrowed.
For example, in order to realize a wide band, while the thickness
of the second waveguide is required to be as small as possible,
actually, there is a limit in decreasing the thickness for strength
or ease of manufacturing.
A frequency at which a different mode (LSE11 mode) is excited due
to the size difference of the transmission line described above is
present in a usage band, leading to an increase in insertion
loss.
As one method of solving this, while a method which increases the
thickness of the second waveguide and moves the frequency, at which
the different mode (LSE11 mode) is generated, to a region lower
than the lower limit of the usage band is considered, if the
thickness of the second waveguide further increases, a frequency at
which a subsequent mode is generated in a high frequency band is
lowered and falls in the usage band, making it difficult to realize
a wider band.
The invention has been accomplished to solve the above-described
problem newly caused by a wide band including a low frequency band,
and an object of the invention is to provide a millimeter waveband
filter which can vary a resonance frequency in a wider band.
Means for Solving the Problem
In order to attain the above-described object, a first aspect of
the invention provides a millimeter waveband filter including a
first waveguide, a second waveguide, the second waveguide being
connected to the first waveguide in a state where one end of the
second waveguide is inserted into the first waveguide to constitute
a transmission line allowing electromagnetic waves in a
predetermined frequency range of a millimeter waveband to propagate
in a TE10 mode, a pair of flat electric wave half mirrors which
have characteristics to transmit a part of the electromagnetic
waves in the predetermined frequency range and to reflect a part of
the electromagnetic waves and are arranged to face each other at an
interval in a state where the first waveguide and the second
waveguide are blocked, and interval variable means for relatively
moving the first waveguide and the second waveguide in a
longitudinal direction of the transmission line in a state
connected together to change the interval between the pair of
electric wave half mirrors. Frequency components centering on a
resonance frequency of a resonator formed between the pair of
electric wave half mirrors are selectively transmitted, the first
waveguide is a square waveguide in which the sectional shape of the
inside of the first waveguide is a rectangular shape such that a
lower limit frequency of a propagatable frequency band of the first
waveguide in the TE10 mode is equal to or less than a lower limit
of the predetermined frequency range, the second waveguide is a
ridge waveguide in which the outside thereof is a rectangular shape
at a predetermined interval with respect to the inside of the first
waveguide and the sectional shape of the inside thereof has a
central portion having a height smaller than the height of both
side portions, and the height and width of each of both side
portions and the central portion are set such that a lower limit
frequency of a propagatable frequency band of the second waveguide
in the TE10 mode is equal to or less than the lower limit frequency
of the propagatable frequency band of the first waveguide in the
TE10 mode.
According to a second aspect of the invention, in the millimeter
waveband filter according to the first aspect of the invention, a
groove which has a length along the longitudinal direction of the
transmission line corresponding to a 1/4 wavelength of
electromagnetic waves to be a leakage prevention target is formed
on the outside of the second waveguide facing the inside of the
first waveguide, and leakage of the electromagnetic waves from the
interval between the outside of the second waveguide and the inside
of the first waveguide is prevented by the groove.
According to a third aspect of the invention, in the millimeter
waveband filter according to the first aspect of the invention, the
pair of electric wave half mirrors respectively have rectangular
plates which have a predetermined thickness and reflect
electromagnetic waves propagating through the transmission line,
and slits which are formed in central portions of the plates along
a long side direction of the plates and transmit a part of the
electromagnetic waves propagating through the transmission line,
and the slits are of a ridge type in which a central portion has a
height smaller than both side portions, and the thickness of the
plates and the height and width of each of both side portions and
the central portion of the slits are set such that transmittance to
the electromagnetic waves propagating through the transmission line
becomes flat in the predetermined frequency range.
According to a fourth aspect of the invention, in the millimeter
waveband filter according to a second aspect of the invention, the
pair of electric wave half mirrors respectively have rectangular
plates which have a predetermined thickness and reflect
electromagnetic waves propagating through the transmission line,
and slits which are formed in central portions of the plates along
a long side direction of the plates and transmit a part of the
electromagnetic waves propagating through the transmission line,
and the slits are of a ridge type in which a central portion has a
height smaller than both side portions, and the thickness of the
plates and the height and width of each of both side portions and
the central portion of the slits are set such that transmittance to
the electromagnetic waves propagating through the transmission line
becomes flat in the predetermined frequency range.
According to a fifth aspect of the invention, in the millimeter
waveband filter according to the first aspect of the invention, the
sectional shape of the inside of the second waveguide is linearly
symmetrical with a center line in a height direction and a center
line in a width direction of the rectangular shape of the outside
of the second waveguide.
According to a sixth aspect of the invention, in the millimeter
waveband filter according to the second aspect of the invention,
the sectional shape of the inside of the second waveguide is
linearly symmetrical with a center line in a height direction and a
center line in a width direction of the rectangular shape of the
outside of the second waveguide.
According to a seventh aspect of the invention, in the millimeter
waveband filter according to the third aspect of the invention, the
sectional shape of the inside of the second waveguide is linearly
symmetrical with a center line in a height direction and a center
line in a width direction of the rectangular shape of the outside
of the second waveguide.
According to an eighth aspect of the invention, in the millimeter
waveband filter according to the fourth aspect of the invention,
the sectional shape of the inside of the second waveguide is
linearly symmetrical with a center line in a height direction and a
center line in a width direction of the rectangular shape of the
outside of the second waveguide.
According to a ninth aspect of the invention, in the millimeter
waveband filter according to the second aspect of the invention,
the groove is provided in a plurality of stages with respect to a
length direction of the transmission line.
According to a tenth aspect of the invention, in the millimeter
waveband filter according to the fourth aspect of the invention,
the groove is provided in a plurality of stages with respect to a
length direction of the transmission line.
According to an eleventh aspect of the invention, in the millimeter
waveband filter according to the sixth aspect of the invention, the
groove is provided in a plurality of stages with respect to a
length direction of the transmission line.
According to a twelfth aspect of the invention, in the millimeter
waveband filter according to the eighth aspect of the invention,
the groove is provided in a plurality of stages with respect to a
length direction of the transmission line.
Advantage of the Invention
In this way, the millimeter waveband filter of the invention
includes the first waveguide, the second waveguide, the second
waveguide being connected to the first waveguide in a state where
one end of the second waveguide is inserted into the first
waveguide to constitute the transmission line allowing
electromagnetic waves in the predetermined frequency range of the
millimeter waveband to propagate in the TE10 mode, the pair of flat
electric wave half mirrors which have characteristics to transmit a
part of the electromagnetic waves in the predetermined frequency
range and to reflect a part of the electromagnetic waves and are
arranged to face each other at the interval in a state where the
first waveguide and the second waveguide are blocked, and the
interval variable means for relatively moving the first waveguide
and the second waveguide in the longitudinal direction of the
transmission line in a state connected together to change the
interval between the pair of electric wave half mirrors. The
frequency components centering on the resonance frequency of the
resonator formed between the pair of electric wave half mirrors are
selectively transmitted, the first waveguide is a square waveguide
in which the sectional shape of the inside of the first waveguide
is a rectangular shape such that the lower limit frequency of the
propagatable frequency band of the first waveguide in the TE10 mode
is equal to or less than the lower limit of the predetermined
frequency range, the second waveguide is a ridge waveguide in which
the outside thereof is a rectangular shape at a predetermined
interval with respect to the inside of the first waveguide and the
sectional shape of the inside thereof has a central portion having
a height smaller than the height of both side portions, and the
height and width of each of both side portions and the central
portion are set such that the lower limit frequency of the
propagatable frequency band of the second waveguide in the TE10
mode is equal to or less than the lower limit frequency of the
propagatable frequency band of the first waveguide in the TE10
mode.
Like the ridge waveguide, a waveguide, in which the height of the
central portion (the sectional shape of the inside) of the
transmission line is set to be smaller than the height of both side
portions, has a characteristic that, even if the area of the
transmission line is smaller than a standard square waveguide,
electromagnetic waves in a low frequency domain can propagate in
the TE10 mode.
Accordingly, even when the second waveguide has the outside so as
to be inserted into the first waveguide at a narrow interval and
has a comparatively large thickness, the height and width of each
of the central portion and both side portions of the transmission
line are selected, whereby it is possible to make the lower limit
frequency of the propagatable frequency band of the second
waveguide smaller than the lower limit frequency of the
propagatable frequency band of the square first waveguide, and
there is no limit on a low frequency band of a usage frequency
range caused by the size difference between the two waveguides,
thereby realizing a wide band.
It is possible to move the frequency generated by the different
mode (LSE11 mode) to a high frequency band, to prevent an increase
in insertion loss in the usage frequency range by the movement of
the frequency to the high frequency band, and to realize a wide
band including a low frequency band.
In the millimeter waveband filter according to the second aspect of
the invention, the groove which has the length along the
longitudinal direction of the transmission line corresponding to
the 1/4 wavelength of the electromagnetic waves to be a leakage
prevention target is formed on the outside of the second waveguide
to prevent leakage of the electromagnetic waves from the interval
between the waveguides. For this reason, it is not necessary to
make the thickness of the second waveguide greater than the 1/4
wavelength of the electromagnetic waves to be a leakage prevention
target, and there is no limit in setting the dimension of the
transmission line of the second waveguide.
In the millimeter waveband filter according to the third aspect of
the invention, the slits which are provided on the plates of the
electric wave half mirrors are of a ridge type in which the height
of the central portion is set to be smaller than the height of both
side portions. For this reason, many parameters including the
thickness of the plates and the width and height of each of both
side portions and the central portion of the slits are selected,
whereby it is possible to set the parameters such that
transmittance to the electromagnetic waves propagating through the
transmission line becomes flat in the predetermined frequency
range, and to realize a wider band as a filter.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A to 1C are basic structure diagrams of a millimeter
waveband filter of the invention.
FIG. 2 is a transmission characteristic diagram of a general square
waveguide.
FIG. 3 is a transmission characteristic diagram of a ridge
waveguide having a size smaller than the waveguide of FIG. 2.
FIGS. 4A and 4B are model diagrams when a length direction of a
groove for electromagnetic wave leakage prevention is changed with
respect to an interval.
FIG. 5 shows a simulation result of a model in which a propagation
direction of electromagnetic waves propagating through an interval
is orthogonal to a length direction of a groove for electromagnetic
wave leakage prevention.
FIG. 6 shows a simulation result of a model in which a propagation
direction of electromagnetic waves propagating through an interval
is parallel to a length direction of a groove for electromagnetic
wave leakage prevention.
FIGS. 7A to 7C are specific structure diagrams of a millimeter
waveband filter of the invention.
FIG. 8 is a transmission characteristic diagram of an electric wave
half mirror in which a height of a slit is constant.
FIG. 9 is a transmission characteristic diagram of an electric wave
half mirror having a ridge slit shown in FIGS. 7A to 7C.
FIG. 10 is a transmission characteristic diagram when a half mirror
interval varies in the millimeter waveband filter shown in FIGS. 7A
to 7C.
MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment of the invention will be described
referring to the drawings.
FIGS. 1A to 1C show the basic structure of a millimeter waveband
filter 20 of the invention. FIG. 1A is a diagram when a part of the
millimeter waveband filter 20 is fractured from the side, FIG. 1B
is a sectional view taken along the line A-A of FIG. 1A, and FIG.
1C is a sectional view taken along the line B-B of FIG. 1A.
As shown in FIGS. 1A to 1C, the millimeter waveband filter 20 has a
first waveguide 22, a second waveguide 24, a pair of electric wave
half mirrors 30A and 30B, and interval variable means 40.
The first waveguide 22 is a square waveguide which has a
transmission line 23 (the inside of the first waveguide) having a
rectangular sectional shape allowing electromagnetic waves in a
predetermined frequency range (for example, 75 to 110 GHz) of a
millimeter waveband to propagate in a TE10 mode (single mode). For
example, a WR-10 waveguide having a size of
w0.times.h0=2.54.times.1.27 mm can be used. In FIGS. 1A to 1C, a
left transmission line 23 and a right transmission line 23' are
separated by the electric wave half mirror 30A. In the basic
structure, while the two transmission lines 23 and 23' have the
same size, the right transmission line 23' connected to an external
circuit may have a standard size corresponding to the WR-10 type,
and the size of the left transmission line 23, into which the
second waveguide 24 is inserted, may be slightly greater than the
standard size (for example, w0'.times.h0'=2.65.times.1.47 mm).
FIG. 2 shows a transmission characteristic (S21) of a WR-10
waveguide having a size of w0.times.h0=2.54.times.1.27 mm which can
be used as the first waveguide 22, and shows a characteristic which
has low loss and is flat in a range from a lower limit frequency of
60 GHz to 160 GHz.
The second waveguide 24 has a transmission line which allows the
electromagnetic waves in the predetermined frequency range (for
example, 75 to 110 GHz) to propagate in the TE10 mode like the
first waveguide 22, and is connected to the first waveguide 22 in a
state where at least one end thereof is inserted into the first
waveguide 22.
As described above, if a square waveguide in which a sectional
shape of a transmission line is a rectangular shape is used as the
second waveguide 24, the transmission line is thinned by the sum of
an interval necessary for a relative movement of the waveguide and
the thickness of the waveguide. Then, as indicated by a dotted line
of FIG. 2, a cutoff frequency of a low frequency band is moved to a
high frequency band and a usable range is narrowed.
Accordingly, in the millimeter waveband filter 20, protrusions 24a
and 24b which protrude in a direction approaching each other from
the centers of the insides on the upper and lower sides (the long
sides of the outside) of the second waveguide 24 are continuously
formed, and the sectional shape of the transmission line 25 (the
inside of the second waveguide) substantially has a H shape. In
this way, a waveguide in which the height h1 of a central portion
25a of the transmission line 25 is set to be smaller than the
height h2 of both side portions 25b and 25c is generally referred
to as a ridge waveguide.
In case of the ridge waveguide, the width w1 and height h1 of the
central portion 25a and the width w2 and height h2 of both side
portions 25b and 25c are selected, whereby it is possible to allow
the electromagnetic waves in the equivalent frequency range to
propagate in the TE10 mode with the sectional shape smaller than
the sectional shape of the transmission line of the standard square
waveguide.
As a dimension example of the second waveguide 24 of this
embodiment, as shown in FIG. 1B, the interval g with respect to the
inside of the first waveguide 22 is set to 30 .mu.m, the size of
the rectangular shape is c.times.d=2.59.times.1.14 mm, the width
and height of the central portion 25a of the transmission line are
w1=0.5 mm and h1=0.27 mm, the width and height of the side portions
25b and 25c of the transmission line are w2=0.72 mm and h2=0.67 mm,
the thickness of the upper and lower sides (the long sides) t1=0.37
mm, the thickness of the right and left sides (the short sides) is
t2=0.325 mm, and the transmission characteristic (S21) of the
waveguide having this shape is as shown in FIG. 3. In FIGS. 1A to
1C, the outside a.times.b of the first waveguide 22 is greater than
the size w0.times.h0 and is arbitrary in a range where strength as
a structure is obtained.
As will be apparent from FIG. 3, even though the sectional shape of
the second waveguide 24 is significantly smaller than the sectional
shape of the transmission line of the standard WR-10 waveguide
shown in FIG. 2, the lower limit frequency is lowered to about 56
GHz.
Accordingly, even if the ridge waveguide is used as the second
waveguide 24, it is possible to allow electromagnetic waves in a
predetermined frequency range (75 to 110 GHz) for use, and to
propagate in the TE10 mode with low loss.
In this way, the two waveguides 22 and 24 having different sizes
are connected and a ridge waveguide is used as the inner waveguide,
whereby transmission lines allowing electromagnetic waves in a
desired frequency range to propagate in the TE10 mode are
continuously formed.
Although an example where the lower limit frequency of the
propagatable frequency band of the second waveguide 24 becomes
lower than the lower limit frequency (in the example of FIG. 2, 60
GHz) of the propagatable frequency band of the first waveguide 22
has been described, the lower limit frequency of the propagatable
frequency band of the second waveguide may be set to be equal to or
less than the lower limit frequency of the propagatable frequency
band of the first waveguide. In this way, the shape of the
transmission line of the second waveguide 24 is set, whereby there
is no limit on the frequency band by the size difference between
the two waveguides and it is possible to realize a wide band even
if the second waveguide 24 having a large thickness is used.
A pair of flat electric wave half mirrors 30A and 30B have a
characteristic to transmit a part of electromagnetic waves in a
predetermined frequency range and to reflect a part of the
electromagnetic waves, and are provided to face each other at a gap
in a state where the transmission line 23 of the first waveguide 22
and the transmission line 25 of the second waveguide 24 are blocked
(also can be read as a state where the first waveguide and the
second waveguide are blocked).
Specifically, each of the electric wave half mirrors 30A and 30B
has a rectangular outside which blocks the transmission line of the
waveguide. One electric wave half mirror 30A is fixed in the
transmission line of the first waveguide 22, and the other electric
wave half mirror 30B is provided at the leading end (the right end
in FIGS. 1A to 1C) of the second waveguide 24.
The electric wave half mirrors 30A and 30B have rectangular plates
31A and 31B which have a predetermined thickness and are made of a
metal material to reflect electromagnetic waves propagating through
the transmission lines, and slits 32A and 32B which are formed in
the central portions of the plates 31A and 31B in the long side
direction of the plates 31A and 31B and transmit a part of the
electromagnetic waves propagating through the transmission
lines.
In regards to the slits 32A and 32B, in FIG. 1C which shows the
basic structure of the filter, although a simple structure in which
the height is constant over the width direction has been shown, as
described below, the height of a portion may be different from
other portions.
The interval variable means 40 relatively moves the first waveguide
22 and the second waveguide 24 in the long side direction of the
transmission lines in a state connected together to vary the gap
between a pair of electric wave half mirrors 30A and 30B, thereby
varying the resonance frequency of the filter determined by the
gap. Although the specific structure of the interval variable means
40 is arbitrary, specifically, the first waveguide 22 having a
large size may be fixed and supported, and the second waveguide 24
may be moved in the longitudinal direction in a state concentric
with the first waveguide 22. As a drive method, a configuration in
which the rotation power of a motor is converted to linear motion
to advance or retreat the second waveguide 24 with respect to the
first waveguide 22, or the like can be used.
In the above-described millimeter waveband filter 20, although the
basic structure in which the ridge waveguide is used as the second
waveguide 24 has been shown, since the second waveguide 24 can have
a large thickness due to a small sectional shape of the
transmission line, it is considered that a groove (choke) for
electromagnetic wave leakage prevention is formed.
Although Patent Document 1 described above describes that the
groove for electromagnetic wave leakage prevention is provided at a
predetermined depth from the inside toward the outside of the outer
waveguide to prevent leakage of electromagnetic waves having a
wavelength corresponding to the depth, in this way, when the groove
is provided on the inside of the outer waveguide, if the inner
waveguide is moved with respect to the outer waveguide so as to
change the resonance wavelength, it is confirmed that the distance
from the outer circumference of the leading end of the inner
waveguide to the groove changes, and the frequency of unnecessary
resonance determined by the distance changes, adversely affecting
the passing characteristic of the filter.
In order to solve this, it is considered that a groove for
electromagnetic wave leakage prevention is provided on the outer
circumference of the inner waveguide to prevent change in distance
to the groove with the movement of the waveguides.
However, the required length as the groove for electromagnetic wave
leakage prevention is around 1 mm which is substantially 1/4 of a
guide wavelength (center wavelength) to be prevented. For this
reason, for example, even if the above-described ridge waveguide is
used as the second waveguide 24, it is not possible to form the
groove at a depth of about 1 mm from the outside toward the inside
(in the above-described numerical example, the thickness t1 of 0.37
mm protrudes).
As a method of solving this, it has been examined whether or not it
is possible to match the length direction representing the
electromagnetic wave leakage prevention action of the groove with
the length direction of the transmission line.
FIG. 4A shows a related art model in which a groove having a length
of 1.1 mm and a width of 0.3 mm is provided so as to be orthogonal
to a gap of 30 .mu.m (transmission line by the interval), and FIG.
4B shows an examination model in which a groove having a length of
1.1 mm and a depth of 0.2 mm is provided along a gap of 30 .mu.m.
The transmission characteristic of the related art model is
obtained as shown in FIG. 5, and the transmission characteristic of
the examination model is obtained as shown in FIG. 6.
In comparison of both in a range of 70 to 120 GHz, it is understood
that the related art model obtains large attenuation compared to
the examination model, and in particular, undergoes steep
attenuation at 94 GHz. However, even in the examination model,
attenuation of 10 dB is obtained in the above-described frequency
range, and if the attenuation is not sufficient, it is possible to
cope with this by forming a groove having the same shape in a
plurality of stages along the length direction of the transmission
line. From this result, it can be confirmed that the groove for
electromagnetic wave leakage prevention can be formed so as to
match the length direction representing the electromagnetic wave
leakage prevention action with the length direction of the
transmission line, and this technique can be sufficiently applied
to the second waveguide 24 having a thickness of around 0.4 mm
described above.
A millimeter waveband filter 20' shown in FIGS. 7A to 7C uses the
examined technique described above, and grooves 60 for
electromagnetic wave leakage prevention are provided on the upper
and lower surfaces close to the leading end of the second waveguide
24 such that the direction representing the electromagnetic wave
leakage prevention action becomes the length direction of the
transmission line. That is, each groove 60 having a length of about
p=1 mm representing the electromagnetic wave leakage prevention
action is formed at a depth of about 0.2 mm. Even in this case,
since the phase of electromagnetic waves propagating and returning
from the edge of the groove 60 close to the half mirror to the edge
away from the half mirror changes by .lamda./2 and input and output
are cancelled (a choke effect in which impedance significantly
increases with respect to leakage electromagnetic waves is
exhibited), an electromagnetic wave leakage prevention effect is
obtained.
While it is expected that the electromagnetic wave leakage
prevention effect by the grooves 60 is attenuation of about 10 dB
from the examination model, as indicated by a dotted line of FIG.
7A, a plurality of grooves 60 are arranged along the length
direction of the transmission line (while two stages are shown in
FIGS. 7A to 7C, the overlapping length of the waveguides may be
extended and three or more stages may be provided), whereby it is
possible to obtain a larger amount of attenuation.
Here, although the grooves 60 are provided on the outsides of the
upper and lower sides (the long sides) of the second waveguide 24
having a high electromagnetic wave leakage prevention effect,
grooves may be also provided on the outsides of the right and left
sides (the short sides).
The grooves 60 should have a depth of about 0.2 mm so as to exhibit
the electromagnetic wave leakage prevention effect (choke effect).
For this reason, as in the related art, when a square waveguide is
used as the second waveguide, as the thickness of the square
waveguide, the dimension (0.3 mm) obtained by adding an amount
corresponding to the depth (0.2 mm) of the grooves 60 and the
required minimum thickness (0.1 mm) as a structure is required, and
the low frequency band is narrowed by an increase in thickness.
As described above, as a result of various examinations on the
reflection characteristic of the electric wave half mirrors 30A and
30B, it has been confirmed that, in the above-described slits
having a shape with a constant height undergo, there is a movement
to transmittance in a desired frequency range.
FIG. 8 shows a transmission characteristic of a single mirror when
the slit 32A (32B) in the plate 31A (31B) having a thickness of 1
mm has a constant height of 50 .mu.m (a transmission characteristic
in a state where the electric wave half mirror is provided in the
transmission line having the same outside as the plate), and shows
a downward convex change in a range of 70 to 115 GHz.
In order to cope with this, in the millimeter waveband filter 20'
shown in FIGS. 7A to 7C, as shown in FIG. 7C, as the slit 32B of
the electric wave half mirror 30B, a ridge type is provided
corresponding to the sectional shape of the transmission line 25 of
the second waveguide 24 such that the height h3 of a central
portion 33a having a width w3 is set to be smaller than the height
h4 of side portions 33b and 33c having a width w4. Though not
shown, the same slit shape applies to the other electric wave half
mirror 30A.
With this slit shape, for example, a transmission characteristic of
a single mirror when the plate thickness is 0.7 mm, the width and
height of the central portion 33a of the slit are w3=0.5 mm and
h3=40 .mu.m, and the width and height of both side portions 33b and
33c are w4=1.02 mm and h4=0.2 mm (a transmission characteristic in
a state where the electric wave half mirror is provided in the
transmission line having the same outside as the plate) is shown in
FIG. 9. As shown in FIG. 9, a flat transmission characteristic over
a wide range of flat 70 to 115 GHz is shown.
The above-described numerical example is a result obtained by
changing the parameters, such as the plate thickness and the width
and height of each of the central portion and both side portions of
the slit, in various ways. Although the numerical values are not
intended to specify the invention, as described above, it has been
confirmed that portions having different heights are provided in
the slit, and changes in characteristic to changes in increased
parameters are recognized to set the parameters, thereby making the
transmission characteristic of the electric wave half mirror
flat.
Though not described in detail, in regards to the tendency of
changes in characteristic to changes in parameters, as the height
h3 of the central portion 33a of the slit increases, transmittance
increases in the entire frequency band, and a noticeable change in
transmission characteristic to a change in the height h4 of both
side portions 33b and 33c does not appear. As the width w3 of the
central portion 33a decreases (that is, the width w4 of both side
portions 33b and 33c increases), transmittance tends to increase in
the entire frequency band. If the gradient of the transmission
characteristic largely changes to change in plate thickness and the
thickness increases in a predetermined range, the gradient of the
transmission characteristic changes from negative to positive.
Accordingly, the plate thickness is set to a value such that the
gradient of the transmission characteristic becomes substantially 0
(the transmission characteristic is substantially parallel to a
frequency axis), and the height h3 and width w3 of the central
portion 33a of the slit are set to values such that preferable
transmittance (for example, about 20 dB) as a half mirror for use
in a resonator, whereby it is possible to make the transmission
characteristic flat. The above-described numerical example shows an
example.
FIG. 10 shows a transmission characteristic when a half mirror
interval u is varied from 3.1 mm to 1.5 mm with 0.04 mm step in the
millimeter waveband filter 20' shown in FIGS. 7A to 7C.
As will be apparent from the drawing, it is understood that a
filter characteristic with substantially constant loss in a
predetermined frequency range of 75 to 110 GHz is obtained through
the use of a small ridge waveguide as the second waveguide 24.
In FIG. 10, a peak which appears near a characteristic of a half
mirror interval u=1.5 mm (equal to or greater than 111 GHz) is
sub-resonance when the half mirror interval u is wide (3.1 mm to
2.9 mm). While a drop near 117 GHz is loss due to the occurrence of
a different mode (LSE11), it is understood that, when a square
waveguide is used as the second waveguide 24, a peak which occurs
in a usage band can be moved to a band higher than the usage
band.
DESCRIPTION OF REFERENCE NUMERALS AND SIGNS
20, 20': millimeter waveband filter, 22: first waveguide, 23, 23':
transmission line (the inside of the first waveguide), 24: second
waveguide, 25: transmission line (the inside of the second
waveguide), 25a: central portion, 25b, 25c: side portion, 30A, 30B:
electric wave half mirror, 31A, 31B: plate, 32A, 32B: slit, 33a:
central portion, 33b: side portion, 40: interval variable means,
60: groove
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